This work focuses on how the nervous system controls human arm stiffness-the resistance to imposed displacements. The long-term goal is to understand how stiffness control is integrated into the ongoing control of movement and how it is used in an adaptive fashion during interactions with the environment. All of the proposed research is related by the common aim of understanding the voluntary control of stiffness. The studies extend from an assessment of basic properties of stiffness control, to studies that examine how stiffness control is more generally involved in the production of movements and to studies that assess how stiffness is used in interactions with the environment. A common procedure is adopted throughout-the use of a robotic manipulator to deliver precise mechanical perturbations to arm. The research will (1) document the basic properties of stiffness control. It will identify the frame of reference for stiffness control, evaluate the durability of stiffness learning, and assess the balance of central and reflex contributions to stiffness (2) The work will characterize the properties of arm stiffness control during movement. It will assess the relationship between stiffness and movement speed, evaluate the role of stiffness control in the production of movements that differ in terms of trajectory requirements and final accuracy. It will also establish the relationship between stiffness as measured under static conditions and stiffness during voluntary movement (3) The studies will also establish properties of stiffness control during interactions with the environment. They will assess the ways in which stiffness is modified when interacting with objects that have different inertial properties and determine the extent to which patterns of stiffness are differentially modified when forces are applied to objects with different surface properties.